1. Field of the Invention
This invention generally relates to a driver impedance control apparatus, and especially to a driver impedance control apparatus and system, which can adjust impedance compensation to any voltage point, and can respectively compensate a pull-up impedance and a pull-down impedance of the driver.
2. Description of Related Art
FIG. 1 is a block diagram of signal transmission between conventional integrated circuits. In general, all of integrated circuits 110 and 130 are welded on a substrate, such as a printed circuit board, and the integrated circuits 110 and 130 transmit data to each other through a conductive path, such as a conduct path 120. When a core circuit 111 of the integrated circuits 110 transmits the data to the integrated circuit 130, a driver 112 transmits the output data of the core circuit 111, through a bonding pad 113, to the conductive path 120. Through the transmission of the conductive path 120, the driver 132, through a bonding pad 133, receives the data output by the integrated circuits 110, and transmits the data to a core circuit 131.
Along with gradually increasing clock of the system, the problem of impedance matching in the signal path is more important. In a high-frequency circuit, much noise can be mixed into the output signal of the integrated circuit 110 after transmitted at an external conductive path 120. Therefore, in general, a terminal resistor 121 is added to an end of the external conductive path 120 which is coupled to a receiver such as the integrated circuit 130, and another end of the terminal resistor 121 is coupled to a terminal voltage Vtt, for reducing noise. Wherein, the level of the terminal voltage Vtt is usually set as half of the power voltage VCC.
Further, in a high-frequency circuit, when an output impedance of the driver is not matched with the load impedance, a part of the output signal of the driver can be reflected and noise is therefore generated. Therefore, the driver 112 must be equipped with a mechanism which can adjust the output impedance, for adjusting the output impedance to match the load impedance. For example, a conventional technology disclosed by U.S. Pat. No. 6,541,996 utilizes an external resistor for determining and compensating the pull-up impedance and the pull-down impedance of the driver.
However, the conventional technology can only compensate the impedance of the VCC/2 voltage level. FIG. 2 is schematically shows a voltage-current characteristic curve of the driver. As shown in FIG. 2, the conventional technology can only compensate the impedance of the VCC/2 voltage level, which is point A in the figure. The above-mentioned conventional technology is not applicable, when compensating the impedance of the driver in other voltage level, such as Vol, is necessary. Recent method is to assume that the voltage-current characteristic of the driver is a linear relation, then the necessary impedance of the voltage level Vol, which is point B′ in the figure, is deduced from the point A. However, in fact, the voltage-current characteristic of the driver is not the linear relation, therefore an error usually exists between the compensated impedance to the voltage level Vol by the conventional technology at the point B′ and the necessary compensating impedance at a point B. That is, the conventional technology cannot correctly make impedance compensation to any of the voltage level.